I remember this being a suggestion from a mod in a poll at one stage.
Can we please have a 12 hour crop? You could even give us two 12 hour options - one for the Grand Farms as well. Looking at the maths for the 5 hour crops, my suggestions are:
12 hour crop - big farm. Cost: $7700. Yield: 540.
12 hour crop - grand farm. Cost: $180,000. Yield: 2000.